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Tiltrotors 1

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FrenchCAD

FrenchCAD

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Feb 8, 2002
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I just read an article about Bell/Augusta BA609 going to be tested in flight and V22 problems.

The tiltrotor technology is very impressive and I wanted to know what you think about it? Do you think we will achieve it? will the transition problems be solved?

Cyril Guichard
Mechanical Engineer
 
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Cyril,

The US Government has too much riding on the V22 to completely write it off. The hydraulic system problem stems from a dumb idea in the first place (using a 5,000 psi system) for a military operations vehicle.

I have a feeling the gov't will spend as much money as required to fix all the problems and get the thing flying. Even if they have to "get creative" about where the find the funds.

I don't see the transition problem as being a program killer. I think there is a lot to be learned on the operational side of the cube, as the transition phase is not intuitive or trivial. As a pilot and an engineer, I see the transition phase as laying claim to a majority of the incidents and accidents, particularly in the Civilian 609. Unless you fly the Harrier, you are already behind the curve.

These are just my opinions and comments.



Regards,
Grant Wittenborn
Aerospace Engineer
 
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I just read a similar article. (In Flight magazine, I think.)

I couldn't believe that anyone would try the whole tiltrotor concept again after the disasterous Osprey, but I guess the theory is sound enough that Bell/Augusta gave it a try.

From the article, it sounds like they can fly the thing pretty good up and down, but are kind of reluctant to take it through the transition mode envelope just yet.

(And that's where the success of the project lies, isn't it?)

It surprised me to see that Bell already has orders for 70 of this aircraft.

 
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Apparently these tilt-rotors are vulnerable to the "Vortex-Ring" state, in which the rotor wash actually circulates BACK into the top of the rotor. You get stuck settling into your own downwash with no way out. It can happen to any helicopter, if the pilot is careless (in a descent?) I don't know what it is about the tilt-rotor that makes the Osprey vulnerable to this state, but it sounds scary, just the same.


STF
 
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Vortex ring state is something, like you said, that is a concern with any helicopter. It is induced when settling with power in a vertical or nearly vertical decent. It happens when the rate of decent is close to the induced velocity of the airflow thru the rotor system. The airflow thru the rotor at the hub tends to reverse direction while the flow toward the outer portion of the rotor disc stays in the downward direction. Unless it's a transitional lift issue, I don't see how this is any different from a conventional helicopter.

BTW, for someone who hasn't read the article mentioned, why is a 5000 psi system a bad idea in military aircraft? The civilian market seems to be grasping at it all out. The JSF is supposedly a 5000 psi aircraft as well. I would think the reduced weight would be a huge plus.
 
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Look at a 100 psi hydraulic line and ask yourself what it would take to pump it up 50 times more. I've never heard of this before, either, but a weight saving is not obvious to me. Perhaps 5000 psi lines can be done rigidly; can a flexible 5000 psi hydraulic line be made?


STF
 
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Here's a couple of exerpts from a Design News article on the A380:

"Toulouse, France –Overall weight and fuel efficiency are important to an aircraft's performance, so Airbus engineers looked past the 3,000-psi systems typically used in large aircraft and designed the first-ever commercial 5,000-psi hydraulic system for use in the A380 aircraft. The new high-pressure system provides the fluid power needed for controlling the aircraft's flaps, rudder, and other control surfaces while reducing the overall weight of the hefty 555-passenger aircraft by 2,500 lb."

Also, in regards to flowrate:

"Designing the system to operate at 5,000 psi allows a 40% reduction in the flow rate compared to a 3,000-psi system built to the scale needed for the A380," says Halat. The jumbo jet contains a large number of actuators, many to drive its large number of control surfaces. The pump and fluid delivery system must meet the maximum flow rate of multiple actuators functioning at one time. In a 3,000-psi system for the A380, the flow rate would require a 70-gpm pump. However, the 5,000-psi system reduces the necessary flow to only 42 gpm. Airbus achieved a 20% weight reduction by going to the higher pressure and using a smaller size displacement pump with lower flow rate.

I'm not sure of flex lines at that pressure however. I really am not at the forefront of fluid power technology, so I can't venture a guess.
 
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Fred,

The initial problem with the 5,000 psi system in the Osprey was thay the high pressure kept bursting gaskets and seals. This set the program back a considerable amount of time. You can't just put 3,000 psi fittings on a 5,000 psi system, and they found this out rather quickly when they ended up with hydraulic fluid all over the place.

On a very large vehicle (A380), the higher pressure system can save you weight, because your hydraulic lines can become smaller in diameter, and you need to carry less fluid. You DO however have to beef up the fittings, gaskets and seals.

On a smaller aircraft, the amount of weight you save by increasing the pressure isn't significant, because you just don't have nearly as much hydraulic tubing on the vehicle (as you would in an A380 sized machine).

There is a reason that 5,000 psi systems are not typically used in small aircraft. Typically, you do not need 5,000 psi to actuate flaps, landing gear, etc. In the JSF however, I believe their thrust vectoring system is hydraulic, and they have a need for high pressure systems.

Like anything in the aerospace world, you are going to pay for performance. The Osprey designers tried to use cheaper low pressure hardware on a high pressure system to save money, but it ended up back firing on them. Rather than replacing the system with low pressure configuration (very expensive), they upgraded the hardware to handle the higher pressure. This means more cost per airplane.

The vortex ring is a fundamental problem in all hovering aircraft, even the Harrier. Re-ingestion of rotor wash will drop you like a brick. There is limited amount of time you can hover, and the time increases as altitude increases (up to I believe 4,000 ft).



Regards,
Grant Wittenborn
Aerospace Engineer
 
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Grant,

There is limited amount of time you can hover...
could you elaborate on that please? I always assumed that hovering can be done indefinitely. My helicopter books (Shapiro, 1956) seem to presume that I already know what the vortex ring-state is.


STF
 
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STF,

When a helicopter starts to hover, initially, as you can picture, the downwash impinges the ground, and spreads out laterally. Over an extended amount of time, the downwash actually "bounces" off the ground and starts to ascend back upwards.

You can then think of the rotor of a helicopter as a vacuum, sucking in ambient air and accelerating it through the rotor plane.

Now that the downwash is ascending, you can imagine that the "vacuum" now wants to suck in the upward traveling air. This is reffered to as recirculation, or vortex ring state.

As recirculation develops, the tip of the rotor blade generates a vortex, that immediately circles up and over the tip, disrupting the flow over the tip of the blade, and actually forcing the blade tip down. Through recirculation, the entire span of the blade loses its lift producing capability, and a sink rate ensues.

The amount of time a vehicle can remain in hover has a lot to do with the altitude of the vehicle, measured in rotor diameters, or in the case of the harrier, the nozzle exit diameter.

In effect, you could have a helicopter sitting on the ramp, producing a small amount of lift. If you let it sit there for long enough, there wouldn't be enough power to produce enough lift to get the helicopter off the ground. You are just recirculating the air through the rotor.

Hope this helps!



Regards,
Grant Wittenborn
Aerospace Engineer
 
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I personally have never encountered vortex ring state and haven't heard it mentioned when talking about a hover, especially in ground effect. In my experience it has been a topic of vertical or nearly vertical motion and abrupt maneuvering. The most common reasons are a very small forward velocity with high power settings. Encountering VRS from a hover is a new one to me.
 
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You are right, it is most likely to occur during a high power descent. Which is most likely to be the cause of the V-22 accidents. However, in extended hover, the recirculation effect through the rotors induces a vortex, much like the high power descent problem.

Regards,
Grant Wittenborn
Aerospace Engineer
 
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"On a very large vehicle (A380), the higher pressure system can save you weight, because your hydraulic lines can become smaller in diameter, and you need to carry less fluid. You DO however have to beef up the fittings, gaskets and seals."

Well, since I'm currently working on the A380 servocontrols, I only can confirm those words.

Cyril Guichard
Mechanical Engineer
 
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They're going slowly :p The 5,000 psi technology is very sensitive. We're currently testing the whole hydraulic system to see its behaviour and to validate servocontrols. Very interesting stuffs btw. You will understand I can't say much about it though :)

Cyril Guichard
Mechanical Engineer
 
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Very interesting! The smaller tiltrotor (because it's lighter) may respond faster to control input and not experience the vortex problem to the same degree. A curiosity question. Would the British BERP or ASP tip on the rotors solve the VJ 22 problem? When it is considered that these paddles provide both the fastest and the slowest rotor blades in the world, the tiltrotor could fly to a higher forward speed as a helicopter before making the transition to "fixed wing" and maybe get around the transition problem. Also, because of the ability to input a much higher angle of attack before the blade stalls, this may improve the hovering condition - what do you think?
ScotCan
 
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The vortex ring state problem for the Osprey is increased because the wing itself blocks the rotor wash. The ideal situation is to have the wing tilt with the rotors which is what I think they done with the civil project.
 
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